Generally, exhaust gas flowing out from an engine through an exhaust manifold is driven into a catalytic converter mounted at an exhaust pipe and is purified therein. After that, noise of the exhaust gas is decreased while passing through a muffler and then the exhaust gas is emitted into the air through a tail pipe. The catalytic converter purifies pollutants contained in the exhaust gas. In addition, a particulate filter for trapping particulate matter (PM) contained in the exhaust gas is mounted in the exhaust pipe.
A denitrification catalyst (DeNOx catalyst) is one type of such a catalytic converter, and purifies nitrogen oxide (NOx) contained in the exhaust gas. If reducing agents such as urea, ammonia, carbon monoxide, and a hydrocarbon (HC) are supplied to the exhaust gas, the NOx contained in the exhaust gas is reduced in the DeNOx catalyst through oxidation-reduction reactions with the reducing agents.
Recently, a lean NOx trap (LNT) catalyst has been used as such a DeNOx catalyst. The LNT catalyst adsorbs the NOx contained in the exhaust gas when an air/fuel ratio is lean, and releases the adsorbed NOx and reduces the released nitrogen oxide and the nitrogen oxide contained in the exhaust gas when the air/fuel ratio is rich.
Since general diesel engines are operated at the lean air/fuel ratio, however, it is required to artificially adjust the air/fuel ratio to be the rich air/fuel ratio (hereinafter, it will be called a ‘regeneration of the LNT’) in order to release the adsorbed NOx from the LNT.
At this time, a part of the NH3 flowing into the LNT and the NH3 generated in the LNT slips from the LNT, and the slipped NH3 can be used as a reducing agent in a selective catalytic reduction (SCR) catalyst. Therefore, if a NH3 mass that slips from the LNT in regeneration is calculated precisely, a mass of the reducing agent that should be injected by a dosing module to be supplied into the SCR catalyst can be precisely calculated.
The above information disclosed in this Background section is only for enhancement of understanding of the background of the disclosure and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.